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Modifying Stax's SRM-T1


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I published a modification for the Stax SRM-T1/T1S/006 in AudioXpress July 2017, which is just out.  The modification adds 5.1 kilohm output safety resistors and cascoded constant current output loads, which with their heatsink fits the space vacated by the plate resistors - see photo.  Spritzer did something similar a few years ago with an SRM-007, adding constant current loads, but he also changed to 6S4A outputs.  

 

Unlike plate resistors, constant current loads burn up practically no signal current, so while the stock amp burns up 9.2 mA signal current in the plate and feedback resistors at its specified 300 VRMS maximum output, the modified amplifier only uses 2.8 mA driving the feedback resistors, leaving much more current available to drive the headphones.  Since the output tubes are much less stressed, distortion is decreased and 2 dB of extra headroom gained. 

 

The modified amp circuit is very similar to a KGST or KGDT.  This is no accident, as the KGST was designed to be “a Stax SRM-007t with no cost or retail considerations,” and the 007t is a higher power T1.

 

The modification lacks the KGST’s regulated power supply, but larger power supply caps have been fitted.  Since the amplifier is fully differential pure class A with current sources or loads at every stage, the current draw is pretty constant, minimizing power supply effects.

 

The modified T1 can drive demanding headphones like the SR-007 Mk I, something that the stock design strains to do.  The relatively flabby bass, somewhat dark tonality and soft treble are largely eliminated.    

 

Replacing all the old electrolytic power supply caps should also be done as routine maintenance.  Parts cost for the modification is about $35, so total parts cost is around $100.  With T1 amplifiers going for $400-$700, this is the best bang for the buck, a good starter project for someone wanting to get into electrostatic amp DIY without going to a full build.

Stax SRM-T1 mod.jpg

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Well I wouldn't call this close to the KGDT/KGST given how fucked the psu is and the weird way Stax adapt the circuits.   The LV supply is something that has to be changed though, it's derived off the HV rails (see the large power resistors in there) and has to go.  I haven't seen the article but I hope you touched on and modified the bias supply as it has no place with modern Stax sets. 

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You're right about the LV supply (Stax did stick in some zener diodes to sort of regulate it), but this was meant to be a simple mod.  It is intended to be bang for buck, and the most significant improvement is the constant current mod.  Yes the power supply could be much better, but the constant current mod is a first order improvement, the power supply is a second order improvement.  

 

The intended audience wasn't the Stax Mafia, it was people who have a Stax amp and want to improve it without going to the expense of a KGST level amp. Or are just getting into Stax and are willing to do a little DIY (or have it done for them). I think more people would be interested in Stax if they could buy a $500 used amp, put $100 and a few hours work into it for some significant improvements, instead of spending a few thousand for the amp.  Not everyone is interested in building from scratch.

 

Actually, I didn't comment on the bias supply as I was not aware that was an issue, but now that I look at the schematic, I see that there should be a 5 megohm resistor after the last capacitor for protection.  Damn!

Edited by JimL
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The cap on the bias supply output is a big no-no these days so a 4M7 resistor after that is what I've usually done.  Good that you added the 5K1 resistors though. 

You are right that this isn't for us so adding a small switcher to power the front end would be overkill. 

One other "wrinkle" to consider are the first and second generation T1's.  Yours is the much more common second (which Stax called "new T1") but I've never had one of the earlier ones.  The circuitry seems to be a bit different but I'm not sure what they changed. 

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Thanks, spritzer.  The article is already published but I can ask them to put in an addendum adding the 5M resistors to the bias supplies.  I haven't seen the first gen T1 either (other than a photo on the internet) but the mods only concern the output stage so I think they should work equally well.

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  • 2 weeks later...
  • 1 month later...
On 6/14/2017 at 10:49 AM, JimL said:

To put it another way, compared to the Stax Mafia, this is like Stax juvenile delinquency.:P

Thanks Jim - as a 'member of the pack' in the JD Stax world (for now at least), I appreciate your publishing the article.

I'm currently using an STM-T1W to drive a pair of ESP-950 headphones as my 'stat option, and while not perfect, with a bit of EQ here and there I'm enjoying the results. Enough so that I'll likely upgrade later, but still learning & experimenting, not ready to buy a new amp just now, so your article was/is welcome info. It's probably time to replace the caps in my T1 anyway, so if I'm going to do that, it seems like I might as well do this at the same time. 

Just to check, do you know whether there were/are gen 1 and gen 2 versions of the SRM-T1W also? Or since the W came later (as I understand), are they based on the gen 2 design? If there are different versions for the W, any tips on how to differentiate?

Lastly, can anyone suggest the best places(s) to post and/or search for reliable & reasonable modders/folks who do good work on 'stat amps and would use good quality  parts? (Sorry if this is a noob/FAQ question, I'd welcome pointers).

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I'm not an expert in the different T1 versions, spritzer is much more knowledgeable on that.  AFAIK, the T1W used a later version of the board  (the early version has the PS caps running across the board from right to left, looking from the front of the amp, whereas the later version has the PS caps on the right side of the board going from front to back).  It's a bit easier to modify the T1 and T1S versions as the heatsink for the constant current sources can be mounted on the side of the case, over the part of the circuit board that contained the output resistors, as shown in the photo I posted.  However, in the T1W you'll have to figure out a way to mount the heatsink close to the location of the output resistors, as that part of the circuit board is not adjacent to the side of the case.

 

Unfortunately, I am not aware of anyone in the business of doing mods on Stax amps, or people who build amps for others, other than spritzer, who does KG builds but not mods, as far as I know.

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28 minutes ago, JimL said:

I'm not an expert in the different T1 versions, spritzer is much more knowledgeable on that.  AFAIK, the T1W used a later version of the board  (the early version has the PS caps running across the board from right to left, looking from the front of the amp, whereas the later version has the PS caps on the right side of the board going from front to back).  It's a bit easier to modify the T1 and T1S versions as the heatsink for the constant current sources can be mounted on the side of the case, over the part of the circuit board that contained the output resistors, as shown in the photo I posted.  However, in the T1W you'll have to figure out a way to mount the heatsink close to the location of the output resistors, as that part of the circuit board is not adjacent to the side of the case.

Unfortunately, I am not aware of anyone in the business of doing mods on Stax amps, or people who build amps for others, other than spritzer, who does KG builds but not mods, as far as I know.

Good info - thanks!

Maybe someone(s) else knows where to look for modders / builders...?  (I'm reasonably sure they exist, based on posts in <other forums>). Suggestions would be welcome.

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  • 3 weeks later...

Having a T1S sitting idle here I decided to have some fun with it.  Drew up a small CCS board, a scrap heatsink and voila...an improved amp is born.  Bias supply fixed, all caps replaced and output resistors fitted.  I ran it a bit hotter than stock so I had to modify the tail resistor on the output cathodes down to 1.5K but that is easy.  DSC_0043-2.thumb.jpg.1d6d4f49faac41161769c85f5ed62d0b.jpg

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so much can go wrong when you are working on something 20+ years old. So many obsolete parts...

The people that know what they are doing probably won't touch stuff they do not own, and people willing to modify your stuff probably are the people you don't want touching it.

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8 minutes ago, kevin gilmore said:

so much can go wrong when you are working on something 20+ years old. So many obsolete parts...

The people that know what they are doing probably won't touch stuff they do not own, and people willing to modify your stuff probably are the people you don't want touching it.

Very much agreed!

I was once asked by a friend to look at and possibly work on his Eico amps and Roger LS3/5A crossovers. After opening them up and look at the parts and the condition of the PCB, etc., my advice to him was "if it is not broken, don't touch it".

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1 hour ago, kevin gilmore said:

so much can go wrong when you are working on something 20+ years old. So many obsolete parts...

The people that know what they are doing probably won't touch stuff they do not own, and people willing to modify your stuff probably are the people you don't want touching it.

 

1 hour ago, mwl168 said:

"if it is not broken, don't touch it".

Wise words...

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Most of the parts in the T1's can be found but yeah, anybody with any sense would never mod these for other people.  The PCB's aren't of good quality and easy enough to damage something beyond repair.  For us it is just fun to muck around with this stuff... 

I for one find it to be cathartic to fix some of the older stuff even if it can try to kill you from time to time.  Looks at the naughty SRA-3S in the corner...  ;)

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7 hours ago, spritzer said:

snip).  Looks at the naughty Kæstur Hákarl festering in the corner...  ;)

FTFY.... Ready to be enjoyed with the Brennivín, Svið, Slátur, and Hangikjöt.

Party time on ice........:wub:

Edited by wink
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